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When you are connecting two servers together you are performing what is called a cluster. A cluster is either two or more servers that are identical in data and configuration. One is primary and other is 2ndary. If one goes down the other takes its place.
RAID 1 is just mirrored drives same data on two drives if one goes down the ther other drive becomes the primary and takes over until replace.
Clusting is more expensive for configureation. Any server IMO should be runing at some sort of raid level.
You are are saying that email went down how did the email go down the app/hardware/network ?
Designing redundancy in server applications such as email is very detailed. But to provide redundance in hardware is much easier. RAID provides Harddisk redunancy and Cluster provides system redunancy.
If your mail app failed neither solution may not even prevented that. could be how the email server was configured or network.
I know does not really answer your question but figuring out how and why went down would be good start and preventing it from doing it again would be first priority if I was maintining that server.
Good Luck
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